Driving method of light emitting device

ABSTRACT

If a potential of a gate electrode of a driving transistor varies after a gray scale signal is inputted into each pixel, a current value of a current supplied to a light emitting element varies so that accurate gray scale display cannot be obtained. In particular, in the case of performing black display, current may flow, which makes clear black display difficult. Accordingly, the invention provides a light emitting device capable of performing accurate gray scale display, and a driving method thereof. According to the invention, a signal for display is inputted plural times within a predetermined timing period, or a writing operation period is lengthened. Consequently, the gate voltage of the transistor is determined after the anode potential of the light emitting element is stabilized, and therefore accurate gray scale display can be performed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a configuration for performing accurategray scale display in a display device having a light emitting element(a light emitting device), and a driving method thereof.

2. Description of the Related Art

In a conventional light emitting device, a pixel configuration as shownin FIG. 9 has been proposed in which a switching element 810 whoseon/off is controlled by a video signal inputted from a signal line 814,a transistor 811 for driving a light emitting element 813, and acapacitor 812 provided between a power source line 815 and a gateelectrode of the transistor 811 to hold a gate-source voltage of thetransistor 811 (see Patent Document 1) are provided.

[Patent Document 1] Japanese Patent Laid-Open No. 2001-343933

It is considered that an equivalent circuit of a light emitting elementdescribed in Patent Document 1 can be shown by a parallel circuitincluding a diode 816 and a capacitor (C_(EL)) in FIG. 9. Operation inthe case where a current value of a current supplied to the lightemitting element 813 varies is described below with reference to FIG. 9.

First, it is assumed that a current at a current value I₀ flowsconstantly into the light emitting element 813. Then, in the case wherethe current value of the current flowing into the light emitting element813 increases from I₀ to I₁, a current value of a current flowing intothe diode 816 does not become I₁ immediately. This is because anincreased amount of the current value of the light emitting element 813is equal to a sum of an increased amount of the current value of thecurrent flowing into the diode 816 and a current value of a currentflowing into the capacitor (C_(EL)). Therefore, the current value of thecurrent flowing into the diode 816 becomes equal to I₁ when the chargingof the capacitor (C_(EL)) is completed.

Meanwhile, assuming that the current at the current value I₀ flowsconstantly into the light emitting element 813 and then the currentvalue decreases from I₀ to I₂, a sum of the current value of the currentflowing into the diode 816 and a current value of a current dischargedfrom the capacitor (C_(EL)) becomes I₂. The current value of the currentflowing into the diode 816 becomes equal to I₂ when the discharging ofthe capacitor (C_(EL)) is completed. In the above-described cases, thetime until which the current value of the constant current flowing intothe diode 816 changes is equal to the time until which changing of apotential between an anode and a cathode of the light emitting element813 is completed, which becomes longer as the size of the capacitor(C_(EL)) is larger and as the changed amount of the current value of thelight emitting element 813 is larger.

The pixel circuit shown in FIG. 9 further includes overlap capacitance(C_(gd)) between a gate electrode and a drain electrode of the drivingtransistor 811 and parasitic capacitance (C_(P)) caused by overlapbetween the gate electrode and the anode and the like depending on thelayout in addition to the capacitor (C_(EL)) between both the electrodesof the light emitting element 813.

At this time, the switching element 810 is turned on, and a currentcorresponding to a gray scale signal inputted into the gate of thetransistor 811 is supplied to the light emitting element 813 and ananode potential thereof changes. However, when the capacitor (C_(EL)) ofthe light emitting element 813 is large and a changed amount of thecurrent value of the current supplied to the light emitting element 813is large, it takes a long time to complete the charging/discharging ofthe capacitor (C_(EL)) and complete the changing of the anode potential.Therefore, there is a case where the changing of the anode potentialdoes not complete in the on-period of the switching element 810.

Then, in the case where the anode potential of the light emittingelement 813 changes (a value of change is ΔV_(A)) after the switchingelement 810 is turned off in FIG. 9, the potential of the gate electrodeof the transistor 811 changes due to capacitive coupling of theparasitic capacitance (C_(P)), the overlap capacitance (C_(gd)), and thecapacitor (C_(S)) 812. A value of change at this time, ΔV_(B) isexpressed by ΔV_(B)=(C_(P)+C_(gd))/(C_(P)+C_(gd)+C_(S))×ΔV_(A).

As set forth above, in the case where the potential of the gateelectrode of the transistor 811 changes after a gray scale signal isinputted into each pixel, there is a problem in that the current valueof the current supplied to the light emitting element 813 changes sothat accurate gray scale display cannot be obtained. In particular, inthe case of performing black display, a current may flow into the lightemitting element so that clear black display cannot be easily performed.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a light emitting devicecapable of performing accurate gray scale display and a driving methodthereof.

In view of the foregoing problem, according to the invention, a signalfor display is inputted plural times within a predetermined timingperiod, or a writing operation period thereof is lengthened. As aresult, the gate voltage is determined after the anode potential of thelight emitting element is stabilized, so that accurate gray scaledisplay can be performed.

A specific mode of the invention is a driving method of a light emittingdevice, in which one frame period is divided into a plurality ofsubframe periods SF1, SF2, . . . , and SFn (n is a positive integer),each subframe period SFn has a writing operation period Ta, and theperiod Te for inputting an erasing signal is provided plural times in atleast one subframe period.

Another mode of the invention is a driving method of a light emittingdevice, in which one frame period is divided into a plurality ofsubframe periods SF1, SF2, . . . , and SFn (n is a positive integer),each subframe period SFn has a writing operation period Ta, and thewriting operation period Ta is provided plural times in at least onesubframe period.

Another mode of the invention is a driving method of a light emittingdevice for performing gray scale display by inputting a video signal andan erasing signal formed of a digital signal, in which a period forinputting the erasing signal is provided longer than a period forinputting the video signal.

A pixel configuration of such a light emitting device comprises aswitching transistor having a source electrode or a drain electrodeconnected to a signal line and a gate electrode connected to a scanline, a driving transistor having a gate electrode connected to theswitching transistor, and a light emitting element which is connected toa source electrode or a drain electrode of the driving transistor.

In addition, the pixel configuration may additionally include an erasingtransistor for discharging a charge corresponding to a gate-sourcevoltage of the driving transistor.

In addition, the pixel configuration may further additionally include atransistor which is connected to the driving transistor in series andthe gate potential of which is fixed.

According to the driving method of the invention, a light emittingdevice capable of performing accurate gray scale display can beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams each showing a driving method of theinvention.

FIGS. 2A and 2B are diagrams showing a timing chart of the invention.

FIGS. 3A and 3B are diagrams showing a timing chart of the invention.

FIGS. 4A and 4B are diagrams each showing a driving method of theinvention.

FIGS. 5A to 5C are diagrams each showing a pixel circuit of theinvention.

FIGS. 6A to 6C are cross-sectional views each showing a pixel structureof the invention.

FIGS. 7A to 7C are cross-sectional views each showing a pixel structureof the invention.

FIGS. 8A to 8F are views of electronic apparatuses of the invention.

FIG. 9 is a diagram showing a pixel configuration of a light emittingdevice.

FIG. 10 is a graph showing experimental results of the invention.

FIGS. 11A and 11B are diagrams showing a timing chart of the invention.

FIGS. 12A and 12B are diagrams showing a timing chart of the invention.

FIGS. 13A and 13B are diagrams showing a timing chart of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Although the invention will be fully described by way of embodimentmodes and an embodiment with reference to the accompanying drawings, itis to be understood that various changes and modifications will beapparent to those skilled in the art. Therefore, unless such changes andmodifications depart from the scope of the invention, they should beconstrued as being included therein. In the drawings for describingembodiment modes and an embodiment, the same portions or portions havingthe same function are denoted by the same reference numerals, and thedescription thereof is not repeated.

EMBODIMENT MODE 1

This embodiment mode describes a driving method in the case where apredetermined signal is inputted plural times.

In FIG. 1A, operation in the case where an erasing signal is inputtedtwo times in a digital gray scale method is shown. First, at apredetermined timing, a digital signal for display (a video signal) isinputted, and then a first erasing signal is inputted after apredetermined time has passed. At this time, if the capacitor (C_(EL))or the parasitic capacitance (C_(P)) exists, a potential of a gateelectrode (a gate potential) of a driving transistor does not relativelybecome 0 only by the first erasing signal so that off operation cannotbe performed; as a result, it is difficult to perform accurate grayscale display and gray scale deviation occurs. In view of this,according to the invention, the erasing signal is inputted again, thatis, a second erasing signal is inputted after a predetermined time haspassed. Accordingly, the gate potential can be relatively made 0 againso that off operation can be performed. As a result, the gray scaledeviation is reduced and accurate gray scale display can be performed.

It is to be noted that although the erasing signal is inputted two timesin FIG. 1A, it may be inputted three or more times. Instead of anerasing signal, the same video signal may be inputted two or more timesas well.

Shown in FIG. 1B are a gate potential of the driving transistor and acurrent flowing into the light emitting element in the case where ananalog signal for display (a gray scale signal) is inputted two times inan analog gray scale method. A dotted line shows a state in the casewhere the gray scale signal is inputted one time as is conventional.

First, a first gray scale signal (SW) is inputted and the gate potentialof the driving transistor becomes a predetermined value. At this time,if the capacitor (C_(EL)) exists, the gate potential graduallydecreases. As a result of this and due to the parasitic capacitance(C_(P)), the current flowing into the light emitting element is notmaintained at a predetermined value but increased gradually; as aresult, the current flowing into the light emitting element ismaintained at a high value as shown by the dotted line so that grayscale deviation occurs. In view of this, according to the invention, thegray scale signal is inputted again, that is, a second gray scale signalis inputted after a predetermined time has passed. Accordingly, the gatepotential of the driving transistor returns to the predetermined valueand the current flowing into the light emitting element also becomes atthe predetermined value.

Note that since the anode potential of the light emitting element isstabilized to a certain extent when the second gray scale signal isinputted, the gate potential of the driving transistor changes lessafter that and thus the current flowing into the light emitting elementincreases less.

Although the gray scale signal is inputted two times in FIG. 1B, theinvention is not limited to this and the gray scale signal may beinputted more than two times.

As set forth above, by the driving method for inputting a predeterminedsignal such as an erasing signal and a gray scale signal plural times, alight emitting device capable of performing accurate gray scale displaycan be provided.

EMBODIMENT MODE 2

Described in this embodiment mode is a timing chart of a digital grayscale method in the case where a period for inputting a video signal anda period for inputting an erasing signal plural times are provided.

One frame period can be divided into a plurality of subframe periodsSF1, SF2, . . . , and SFn (n is a positive integer). FIG. 2A is a timingchart in the case where one frame period is divided into three subframeperiods (SF1, SF2, and SF3) to perform 6-gray scale display, in which anerasing signal is inputted two times in the subframe period SF3. FIG. 2Bis a timing chart focusing on a scan line of the i-th row.

The subframe periods (SF1, SF2, and SF3) have writing operation periodsfor inputting a video signal (Ta1, Ta2, and Ta3) (also referred to asperiods for inputting a writing signal) and light emitting periods forperforming light emission depending on the written video signal (Ts1,Ts2, and Ts3) respectively. The length of the light emitting periods isset to satisfy Ts1:Ts2:Ts3=2²:2¹:2⁰.

In the shortest subframe period SF3, periods for inputting an erasingsignal two times Te3(1) and Te3(2) are provided. By providing theperiods for inputting an erasing signal two times Te3(1) and Te3(2), thegate potential of the driving transistor can be accurately determinedeven if the capacitor (C_(EL)) exists. Accordingly, accurate gray scaledisplay can be performed.

It is to be noted that by inputting an erasing signal in the subframeperiod SF3, input of a writing signal in the subframe period SF1 of thenext frame can immediately start, which leads to a high duty ratio.

The driving method in this embodiment mode can be realized by using apixel circuit including an erasing transistor for discharging a chargecorresponding to a gate-source voltage of the driving transistor. Forexample, a pixel circuit shown in FIG. 5B which is described later canbe used.

Note that although the two periods for inputting an erasing signal areprovided in the subframe period SF3 in this embodiment mode, theinvention is not limited to this; for example, three or more periods forinputting the erasing signal may be provided and such a period may beprovided in the period other than the subframe period SF3.Alternatively, a plurality of the writing operation periods may beprovided in order to input the same writing signal plural times. Thatis, according to the invention, the difficulty in performing accurategray scale display due to the capacitor (C_(EL)) is solved by providinga plurality of inputting periods for inputting a predetermined signalplural times.

EMBODIMENT MODE 3

Described in this embodiment mode is a timing chart of a digital grayscale method in the case where a plurality of writing operation periods(also referred to as periods for inputting a writing signal) isprovided.

One frame period can be divided into a plurality of subframe periodsSF1, SF2, . . . , and SFn (n is a positive integer). FIG. 3A is a timingchart in the case where one frame period is divided into three subframeperiods (SF1, SF2, and SF3) to perform 6-gray scale display, in which aperiod for applying a reverse voltage is provided. FIG. 3B is a timingchart focusing on a scan line of the i-th row.

The subframe periods (SF1, SF2, and SF3) include writing operationperiods (Ta1(W), Ta2(W), and Ta3(W)) and light emitting periods forperforming light emission depending on the written signal (Ts1, Ts2, andTs3) respectively. The length of the light emitting periods is set tosatisfy Ts1:Ts2:Ts3=2²:2¹:2⁰. In the shortest subframe period SF3, aperiod for inputting an erasing signal (Ta3(E)) is provided. The writtensignal is erased in the period for inputting an erasing signal.

In the longest subframe period SF1, for example, two writing operationperiods Ta1 are provided (referred to as Ta1(1) and Ta1(2),respectively). In the periods Ta1(1) and Ta1(2), periods for inputting avideo signal Ta1(W)(1) and Ta1(W)(2) are provided. The video signal iswritten in the first writing operation period Ta1 (1) (referred to as aperiod Ta1(W)(1)), and the video signal is also written in the secondwriting operation period Ta1(2) (referred to as a period Ta1(W)(2)). Inthis manner, the video signal can be written plural times. Accordingly,the gate potential of the driving transistor can be accuratelycontrolled even if the capacitor (C_(EL)) exists.

The driving method in this embodiment mode can be realized without anerasing transistor for discharging a charge corresponding to agate-source voltage of the driving transistor. A high aperture ratio ofa pixel portion can be obtained because the erasing transistor is notrequired. For example, a pixel circuit shown in FIG. 5A which isdescribed later can be used in a pixel portion. However, a drivercircuit for providing a writing operation period Ta(W) and a period forinputting an erasing signal Ta(E) is required.

In a period for applying a reverse voltage (FRB), a reverse voltage isapplied to a light emitting element (RB). Before the period for applyinga reverse voltage, a period for inputting the erasing signal (Ta(E)) isprovided in which data written in the subframe period immediately beforethe period for inputting the erasing signal, namely in SF3 in thisembodiment mode is sequentially erased. This is because since thereverse voltage is applied to all the light emitting elements at thesame time, some elements may emit light when the reverse voltage isapplied if the data remains. By applying such a reverse voltage to thelight emitting element, a defect state of the light emitting element canbe improved and the reliability thereof can be improved. The lightemitting element, in particular, may have an initial defect that ananode and a cathode thereof are short-circuited due to adhesion offoreign substances, some pinholes that are produced by minuteprojections of the anode or the cathode, or nonuniformity of anelectroluminescent layer thereof. When such an initial defect occurs,light emission/non-light emission in accordance with a signal is notperformed and almost all currents flow into the short-circuited portion.Consequently, favorable image display cannot be performed. In addition,such a defect may occur in an arbitrarily pixel.

As according to this embodiment mode, by applying a reverse voltage tothe light emitting element, a current flowing locally into theshort-circuited portion generates heat to oxidize or carbonize theshort-circuited portion. As a result, the short-circuited portion can beinsulated and a current flows into the region other than the insulatedportion so that normal operation as the light emitting element can beobtained. By applying a reverse voltage, the initial defect can beresolved as described above. Note that the insulation of theshort-circuited portion is preferably performed before shipping.

Further, not only an initial defect, but another defect might occur withtime in which the anode and the cathode are short-circuited. Such adefect is also called a progressive defect. However, as according tothis embodiment mode, by applying a reverse voltage to the lightemitting element regularly, the progressive defect can also be resolvedand normal operation can be performed.

Furthermore, applying a reverse voltage can also prevent image burn-in.The image burn-in occurs depending on the degradation state of a lightemitting element; the degradation state can be reduced by applying areverse voltage. Therefore, image burn-in can be prevented.

Such a degradation progresses largely in the initial stage; the progressspeed of degradation decreases with time. That is, a once-degraded lightemitting element is less easily degraded with time. As a result, a lightemitting element which has degraded in the initial stage and a lightemitting element which has degraded with time are mixed, and variationoccurs in the degradation states of the light emitting elements. In viewof this, by making all light emitting elements emit light beforeshipping, when an image is not displayed or the like, the light emttingelement which has not degraded in the initial stage yet is degraded, bywhich the degradation states can be averaged. Constitution for makingall light emitting elements emit light as described above may beadditionally provided in the light emitting device.

It is to be noted that the period for applying a reverse voltage is notlimited to that shown in FIGS. 3A and 3B; for example, the period may beprovided at the start of one frame period. In addition, the period forapplying a reverse voltage is not necessarily required to be provided ineach frame period.

The period for applying a reverse voltage may be provided in the case ofthe timing chart shown in FIGS. 2A and 2B as well.

Although two writing operation periods are provided in the subframeperiod SF1 in this embodiment mode, the invention is not limited tothis; for example, more than two writing operation periods may beprovided. In addition, a plurality of writing operation periods may beprovided in another subframe period. That is, according to theinvention, the difficulty in performing accurate gray scale display dueto the capacitor (C_(EL)) is solved by providing a plurality of periodsfor inputting a predetermined signal.

FIGS. 13A and 13B is a timing chart in the case where a plurality ofperiods for inputting an erasing signal Ta(E), for example two periods,are provided.

FIG. 13A is a timing chart in the case where one frame period is dividedinto three subframe periods (SF1, SF2, and SF3) to perform 6-gray scaledisplay, in which a period for applying a reverse voltage is provided.FIG. 13B is a timing chart focusing on a scan line of the i-th row.

The subframe periods (SF1, SF2, and SF3) include writing operationperiods (Ta1(W), Ta2(W), and Ta3(W)) and light emitting periods forperforming light emission depending on the written signal (Ts1, Ts2, andTs3) respectively. The length of the light emitting periods is set tosatisfy Ts1:Ts2:Ts3=2²:2¹:2⁰.

In the shortest subframe period SF3, a video signal is inputted in thewriting operation period Ta3(W) and a erasing signal is inputted in thetwo periods for inputting the erasing signal Ta3(E)(2) and Ta3(E)(3).Accordingly, the gate potential of the driving transistor can beaccurately controlled even if the capacitor (C_(EL)) exists.

By providing a period for applying a reverse voltage to the lightemitting element in FIGS. 13A and 13B similarly to FIGS. 3A and 3B, theprogressive defect can be resolved as describe above.

EMBODIMENT MODE 4

Described in this embodiment mode is a driving method in the case wheretime for inputting a predetermined signal is lengthened.

In FIG. 4A, operation of a digital gray scale method in the case where aperiod for inputting an erasing signal is longer than a period forinputting a video signal is shown. By lengthening the period forinputting an erasing signal, change of the gate potential after a videosignal is inputted is suppressed, and besides, micro-light emission oflight emitting elements after the erasing operation is performed can bereduced. Consequently, accurate black display can be performed.

In FIG. 4B, operation of an analog gray scale method in the case where aperiod for inputting a gray scale signal is lengthened is shown. Inparticular, in the case of low-gray scale display, presence of thecapacitor (C_(EL)) affects largely since a current flowing into thelight emitting element is small; therefore, the period for inputting agray scale signal may be preferably longer in the case of low-gray scaledisplay than in the case of high-gray scale display.

It is to be noted that the period for inputting a gray scale signal isdetermined depending on the frame frequency, the number of pixels, andthe number of columns into which the signal is inputted at the same time(hereinafter referred to as the number of parallel columns of writing).The frame frequency and the number of pixels are related to displayperformance, and as they are larger, the period for inputting a grayscale signal becomes shorter.

The number of parallel columns of writing is related to a hardwarestructure, and as the number of parallel columns of writing is smaller,the period for inputting a gray scale signal becomes shorter. Note thatin a line sequential writing method, the number of parallel columns ofwriting is equal to the number of horizontal pixels.

As described above, in accordance with increase in the number of pixelsdue to improvement in the image quality, the period for inputting a grayscale signal becomes shorter.

Meanwhile, a current flowing into a light emitting element is decreaseddue to improvement in the efficiency of the light emitting element sothat the period for inputting a gray scale signal is required to belengthened.

Accordingly, in the case of low-gray scale display in particular, theperiod for inputting a gray scale signal is lengthened, which can beachieved by decreasing the frame frequency. Consequently, gray scaledeviation is decreased and accurate gray scale display can be performed.

As set forth above, by a driving method in which a period for inputtinga predetermined signal is lengthened, a light emitting device forperforming accurate gray scale display can be provided.

EMBODIMENT MODE 5

Described in this embodiment mode is a timing chart of a digital grayscale method in the case where time for inputting a predetermined signalis lengthened.

In the case where time for inputting a predetermined signal islengthened, the timing chart in which one frame period is divided into aplurality of subframe periods as shown in FIGS. 2A and 2B, or the timingchart in which a reverse voltage is applied as shown in FIGS. 3A and 3Bcan be employed as well.

For example, in the timing chart as shown in FIGS. 11A and 11B, oneperiod for inputting an erasing signal Te3(1) is provided in thesubframe period SF3 and the period Te3(1) is lengthened (see FIGS. 11Aand 11B). Alternatively, for example, in the timing chart as shown inFIGS. 12A and 12B, the writing operation period Ta3(W) and the periodfor inputting an erasing signal Ta3(E) provided in the subframe periodSF3 are lengthened (see FIGS. 12A and 12B).

The other structure of the timing chart in this embodiment mode is thesame as those in FIGS. 2A and 2B and FIGS. 3A and 3B, and therefore,description thereof is omitted here.

EMBODIMENT MODE 6

In this embodiment mode, an equivalent circuit diagram of a pixelincluded in a light emitting device of the invention is described withreference to FIGS. 5A to 5C.

FIG. 5A is an example of an equivalent circuit diagram of a pixel, whichincludes a signal line 6114, a power supply line 6115, a scan line 6116,a light emitting element 6113, a switching transistor 6110, a drivingtransistor for driving the light emitting element 6111, and a capacitor6112. The signal line 6114 is inputted with a video signal by a signalline driver circuit. On/off of the switching transistor 6110 iscontrolled by the video signal. The switching transistor 6110 cancontrol supply of potential of the video signal to a gate of the drivingtransistor 6111 in accordance with a selection signal inputted into thescan line 6116. The driving transistor 6111 can control current supplyto the light emitting element 6113 in accordance with the potential ofthe video signal. The capacitor 6112 can hold a gate-source voltage ofthe driving transistor 6111. It is to be noted that although thecapacitor 6112 is provided in FIG. 5A, it is not required to be providedif the gate capacitance of the driving transistor 6111 or otherparasitic capacitance can substitute.

FIG. 5B is an equivalent circuit diagram of a pixel in which an erasingtransistor 6118 and a scan line 6119 are additionally provided in thepixel shown in FIG. 5A. By the erasing transistor 6118, respectivepotential of a gate and a source of the transistor 6111 can be equal toeach other to make no current flow into the light emitting element 6113forcibly. Therefore, a subframe period can be shorter than a period forinputting a video signal into all pixels. Consequently, the duty ratiocan be improved.

FIG. 5C is an equivalent circuit diagram of a pixel in which atransistor 6125 and a wiring 6126 are additionally provided in the pixelshown in FIG. 5B. Gate potential of the transistor 6125 is fixed by thewiring 6126. In addition, the driving transistor 6111 and the transistor6125 are connected in series between the power source line 6115 and thelight emitting element 6113. Therefore, in FIG. 5C, the transistor 6125controls the amount of current supplied to the light emitting element6113 while the driving transistor 6111 controls whether the current issupplied or not to the light emitting element 6113.

It is to be noted that a configuration of a pixel circuit in the lightemitting device of the invention is not limited to those described inthis embodiment mode. This embodiment mode can be freely combined withthe above-described embodiment modes.

EMBODIMENT MODE 7

In this embodiment mode, a sectional structure of a pixel in which adriving transistor is a p-channel thin film transistor (TFT) isdescribed with reference to FIGS. 6A to 6C. Note that, in the invention,one of an anode and a cathode of a light emitting element, of which thepotential can be controlled by a transistor is referred to as a firstelectrode, and the other is referred to as a second electrode. Althoughdescription is made on the case where the first electrode is the anodeand the second electrode is the cathode in FIGS. 6A to 6C, it ispossible that the first electrode is the cathode and the secondelectrode is the anode as well.

FIG. 6A is a sectional view of a pixel in which a TFT 6001 is a p-typeand light emitted from a light emitting element 6003 is extracted from afirst electrode 6004 side. The first electrode 6004 of the lightemitting element 6003 is electrically connected to the TFT 6001 in FIG.6A.

The TFT 6001 is covered with an interlayer insulating film 6007, and abank 6008 having an opening is formed over the interlayer insulatingfilm 6007. In the opening of the bank 6008, the first electrode 6004 ispartially exposed, and the first electrode 6004, an electroluminescentlayer 6005 and a second electrode 6006 are stacked in this order.

The interlayer insulating film 6007 can be formed by using an organicresin film, an inorganic insulating film, or an insulating filmcontaining siloxane as a starting material and having Si—O—Si bonds(hereinafter referred to as a “siloxane insulating film”). Siloxanecorresponds to a resin having Si—O—Si bonds. Siloxane is composed of askeleton formed by the bond of silicon (Si) and oxygen (O), in which anorganic group containing at least hydrogen (such as an alkyl group oraromatic hydrocarbon) is included as a substituent. Alternatively, afluoro group may be used as the substituent. Further alternatively, afluoro group and an organic group containing at least hydrogen may beused as the substituent. The interlayer insulating film 6007 may also beformed using a so-called low dielectric constant material (low-kmaterial).

The bank 6008 can be formed by using an organic resin film, an inorganicinsulating film, or a siloxane insulating film. In the case of anorganic resin film, for example, acrylic, polyimide, polyamide, or thelike can be used. In the case of an inorganic insulating film, siliconoxide, silicon nitride oxide, or the like can be used. Preferably, thebank 6008 is formed by using a photosensitive organic resin film and hasan opening on the first electrode 6004 which is formed such that theside face thereof has a slope with a continuous curvature, which canprevent the first electrode 6004 and the second electrode 6006 frombeing connected to each other.

The first electrode 6004 is formed by using a material or with athickness to transmit light, and by using a material suitable for beingused as an anode. For example, the first electrode 6004 can be formed byusing a light-transmissive conductive oxide such as indium tin oxide(ITO), zinc oxide (ZnO), indium zinc oxide (IZO), and gallium-doped zincoxide (GZO). Alternatively, the first electrode 6004 may be formed byusing indium tin oxide containing silicon oxide (hereinafter referred toas ITSO) or a mixture of indium oxide containing silicon oxide and 2 to20 atomic % of zinc oxide (ZnO). Further alternatively, other than theaforementioned light-transmissive conductive oxide, the first electrode6004 may be formed by using, for example, a single-layer film of one ormore of TiN, ZrN, Ti, W, Ni, Pt, Cr, Ag, Al and the like, astacked-layer structure of a titanium nitride film and a film mainlycontaining aluminum, or a three-layer structure of a titanium nitridefilm, a film mainly containing aluminum and a titanium nitride film;however, when employing a material other than the light-transmissiveconductive oxide, the first electrode 6004 is formed thick enough totransmit light (preferably about 5 to 30 nm).

The second electrode 6006 is formed by using a material or with athickness to reflect or shield light, and can be formed by using ametal, an alloy, an electrically conductive compound each having a lowwork function, or a mixture of them. Specifically, an alkali metal suchas Li and Cs, an alkaline earth metal such as Mg, Ca and Sr, an alloycontaining such metals (Mg:Ag, Al:Li, Mg:In, or the like), a compound ofsuch metals (calcium fluoride such as CaF₂ or calcium nitride such asCa₃N₂), or a rare-earth metal such as Yb and Er can be employed. In thecase where an electron injection layer is provided, a conductive layersuch as an Al layer can be employed as well.

The electroluminescent layer 6005 is structured by a single layer or aplurality of layers. In the case of a plurality of layers, the layerscan be classified into a hole injection layer, a hole transportinglayer, a light emitting layer, an electron transporting layer, anelectron injection layer and the like in terms of the carriertransporting property. When the electroluminescent layer 6005 has any ofthe hole injection layer, the hole transporting layer, the electrontransporting layer and the electron injection layer in addition to thelight emitting layer, the hole injection layer, the hole transportinglayer, the light emitting layer, the electron transporting layer and theelectron injection layer are stacked in this order on the firstelectrode 6004. Note that the boundary between the layers is notnecessarily distinct, and the boundary may not be distinguished clearlysince the materials forming the respective layers are partially mixed.Each of the layers can be formed by using an organic material or aninorganic material. As for an organic material, any of the high, mediumand low molecular weight materials can be employed. Note that the mediummolecular weight material means a low polymer in which the repeatednumber of structural units (the degree of polymerization) is about 2 to20. There is no clear distinction between the hole injection layer andthe hole transporting layer, and the hole transporting property (holemobility) is particularly significant in both of them. The holeinjection layer is in contact with the anode while a layer in contactwith the hole injection layer is called a hole transporting layer to bedistinguished for convenience. The same can be applied to the electrontransporting layer and the electron injection layer; a layer in contactwith the cathode is called an electron injection layer while a layer incontact with the electron injection layer is called an electrontransporting layer. The light emitting layer may additionally have afunction of the electron transporting layer, and thus may be called alight emitting electron transporting layer.

In the pixel shown in FIG. 6A, light emitted from the light emittingelement 6003 can be extracted from the first electrode 6004 side asshown by a hollow arrow.

Next, FIG. 6B is a sectional view of a pixel in which a TFT 6011 is ap-type and light emitted from a light emitting element 6013 is extractedfrom a second electrode 6016 side. A first electrode 6014 of the lightemitting element 6013 is electrically connected to the TFT 6011 in FIG.6B. On the first electrode 6014, an electroluminescent layer 6015 andthe second electrode 6016 are stacked in this order.

The first electrode 6014 is formed by using a material or with athickness to reflect or shield light, and formed by using a materialsuitable for being used as an anode. For example, the first electrode6014 may be formed by using a single-layer film of one or more of TiN,ZrN, Ti, W, Ni, Pt, Cr, Ag, Al and the like, a stacked-layer structureof a titanium nitride film and a film mainly containing aluminum, athree-layer structure of a titanium nitride film, a film mainlycontaining aluminum and a titanium nitride film, or the like.

The second electrode 6016 is formed by using a material or with athickness to transmit light, and can be formed by using a metal, analloy, an electrically conductive compound each having a low workfunction, or a mixture of them. Specifically, an alkali metal such as Liand Cs, an alkaline earth metal such as Mg, Ca and Sr, an alloycontaining such metals (Mg:Ag, Al:Li, Mg:In, or the like), a compound ofsuch metals (calcium fluoride such as CaF₂ or calcium nitride such asCa₃N₂), or a rare-earth metal such as Yb and Er can be employed. In thecase where an electron injection layer is provided, a conductive layersuch as an Al layer can be employed as well. Moreover, the secondelectrode 6016 is formed thick enough to transmit light (preferablyabout 5 to 30 nm). Note that the second electrode 6016 may also beformed by using a light-transmissive conductive oxide such as indium tinoxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), andgallium-doped zinc oxide (GZO). Further alternatively, indium tin oxidecontaining silicon oxide (ITSO) or a mixture of indium oxide containingsilicon oxide and 2 to 20 atomic % of zinc oxide (ZnO) may be employed;in the case of employing a light-transmissive conductive oxide, anelectron injection layer is preferably provided in theelectroluminescent layer 6015.

The electroluminescent layer 6015 can be formed similarly to theelectroluminescent layer 6005 shown in FIG. 6A.

In the pixel shown in FIG. 6B, light emitted from the light emittingelement 6013 can be extracted from the second electrode 6016 side asshown by a hollow arrow.

FIG. 6C is a sectional view of a pixel in which a TFT 6021 is a p-typeand light emitted from a light emitting element 6023 is extracted fromboth a first electrode 6024 side and a second electrode 6026 side. Thefirst electrode 6024 of the light emitting element 6023 is electricallyconnected to the TFT 6021 in FIG. 6C. On the first electrode 6024, anelectroluminescent layer 6025 and the second electrode 6026 are stackedin this order.

The first electrode 6024 can be formed similarly to the first electrode6004 shown in FIG. 6A while the second electrode 6026 can be formedsimilarly to the second electrode 6016 shown in FIG. 6B. Theelectroluminescent layer 6025 can be formed similarly to theelectroluminescent layer 6005 shown in FIG. 6A.

In the pixel shown in FIG. 6C, light emitted from the light emittingelement 6023 can be extracted from both the first electrode 6024 sideand the second electrode 6026 side as shown by hollow arrows.

This embodiment mode can be freely combined with the above-describedembodiment modes.

EMBODIMENT MODE 8

In this embodiment mode, a sectional structure of a pixel in which atransistor for controlling current supply to a light emitting element isan n-channel TFT is described with reference to FIGS. 7A to 7C. Notethat although a first electrode is a cathode while a second electrode isan anode in FIGS. 7A to 7C, it is possible that the first electrode isthe anode while the second electrode is the cathode as well.

FIG. 7A is a sectional view of a pixel in which a TFT 6031 is an n-typeand light emitted from a light emitting element 6033 is extracted from afirst electrode 6034 side. The first electrode 6034 of the lightemitting element 6033 is electrically connected to the TFT 6031 in FIG.7A. On the first electrode 6034, an electroluminescent layer 6035 and asecond electrode 6036 are stacked in this order.

The first electrode 6034 is formed by using a material or with athickness to transmit light, and can be formed by using a metal, analloy, an electrically conductive compound each having a low workfunction, or a mixture of them. Specifically, an alkali metal such as Liand Cs, an alkaline earth metal such as Mg, Ca and Sr, an alloycontaining such metals (Mg:Ag, Al:Li, Mg:In, or the like), a compound ofsuch metals (calcium fluoride such as CaF₂ or calcium nitride such asCa₃N₂), or a rare-earth metal such as Yb and Er can be employed. In thecase where an electron injection layer is provided, a conductive layersuch as an Al layer can be employed as well. Then, the first electrode6034 is formed thick enough to transmit light (preferably about 5 to 30nm). Furthermore, a light-transmissive conductive layer may beadditionally formed using a light-transmissive conductive oxide so as tocontact the top or bottom of the aforementioned conductive layer havinga thickness enough to transmit light in order to suppress the sheetresistance of the first electrode 6034. Note that the first electrode6034 may also be formed by using only a conductive layer employing alight-transmissive conductive oxide such as indium tin oxide (ITO), zincoxide (ZnO), indium zinc oxide (IZO), and gallium-doped zinc oxide(GZO). Further alternatively, indium tin oxide containing silicon oxide(ITSO) or a mixture of indium oxide containing silicon oxide and 2 to 20atomic % of zinc oxide (ZnO) may be employed; in the case of employing alight-transimmsive conductive oxide, an electron injection layer ispreferably provided in the electroluminescent layer 6035.

The second electrode 6036 is formed by using a material or with athickness to reflect or shield light, and is formed by using a materialsuitable for being used as an anode. For example, the second electrode6036 may be formed by using a single-layer film of one or more of TiN,ZrN, Ti, W, Ni, Pt, Cr, Ag, Al and the like, a stacked-layer structureof a titanium nitride film and a film mainly containing aluminum, athree-layer structure of a titanium nitride film, a film mainlycontaining aluminum and a titanium nitride film, or the like.

The electroluminescent layer 6035 can be formed similarly to theelectroluminescent layer 6005 shown in FIG. 6A. When theelectroluminescent layer 6035 has any of a hole injection layer, a holetransporting layer, an electron transporting layer and an electroninjection layer in addition to a light emitting layer, the electroninjection layer, the electron transporting layer, the light emittinglayer, the hole transporting layer and the hole injection layer arestacked in this order on the first electrode 6034.

In the pixel shown in FIG. 7A, light emitted from the light emittingelement 6033 can be extracted from the first electrode 6034 side asshown by a hollow arrow.

Next, FIG. 7B is a sectional view of a pixel in which a TFT 6041 is ann-type and light emitted from a light emitting element 6043 is extractedfrom a second electrode 6046 side. A first electrode 6044 of the lightemitting element 6043 is electrically connected to the TFT 6041 in FIG.7B. On the first electrode 6044, an electroluminescent layer 6045 andthe second electrode 6046 are stacked in this order.

The first electrode 6044 is formed by using a material or with athickness to reflect or shield light, and can be formed by using ametal, an alloy, an electrically conductive compound each having a lowwork function, or a mixture of them. Specifically, an alkali metal suchas Li and Cs, an alkaline earth metal such as Mg, Ca and Sr, an alloycontaining such metals (Mg:Ag, Al:Li, Mg:In, or the like), a compound ofsuch metals (calcium fluoride such as CaF₂ or calcium nitride such asCa₃N₂), a rare-earth metal such as Yb and Er, or the like can beemployed. In the case where an electron injection layer is provided, aconductive layer such as an Al layer can be employed as well.

The second electrode 6046 is formed by using a material or with athickness to transmit light, and by using a material suitable for beingused as an anode. For example, the second electrode 6046 can be formedby using a light-transmissive conductive oxide such as indium tin oxide(ITO), zinc oxide (ZnO), indium zinc oxide (IZO), and gallium-doped zincoxide (GZO). Alternatively, the second electrode 6046 may be formed byusing indium tin oxide containing silicon oxide (ITSO) or a mixture ofindium oxide containing silicon oxide and 2 to 20 atomic % of zinc oxide(ZnO). Further alternatively, other than the aforementionedlight-transimissive conductive oxide, the second electrode 6046 may beformed by using, for example, a single-layer film of one or more of TiN,ZrN, Ti, W, Ni, Pt, Cr, Ag, Al and the like, a stacked-layer structureof a titanium nitride film and a film mainly containing aluminum, athree-layer structure of a titanium nitride film, a film mainlycontaining aluminum and a titanium nitride film, or the like; however,when employing a material other than the light-transmissive conductiveoxide, the second electrode 6046 is formed thick enough to transmitlight (preferably about 5 to 30 nm).

The electroluminescent layer 6045 can be formed similarly to theelectroluminescent layer 6035 shown in FIG. 7A.

In the pixel shown in FIG. 7B, light emitted from the light emittingelement 6043 can be extracted from the second electrode 6046 side asshown by a hollow arrow.

FIG. 7C is a sectional view of a pixel in which a TFT 6051 is an n-typeand light emitted from a light emitting element 6053 is extracted fromboth a first electrode 6054 side and a second electrode 6056 side. Thefirst electrode 6054 of the light emitting element 6053 is electricallyconnected to the TFT 6051 in FIG. 7C. On the first electrode 6054, anelectroluminescent layer 6055 and the second electrode 6056 are stackedin this order.

The first electrode 6054 can be formed similarly to the first electrode6034 shown in FIG. 7A while the second electrode 6056 can be formedsimilarly to the second electrode 6046 shown in FIG. 7B. Theelectroluminescent layer 6055 can be formed similarly to theelectroluminescent layer 6035 shown in FIG. 7A.

In the pixel shown in FIG. 7C, light emitted from the light emittingelement 6053 can be extracted from both the first electrode 6054 sideand the second electrode 6056 side as shown by hollow arrows.

This embodiment mode can be freely combined with the above-describedembodiment modes.

EMBODIMENT MODE 9

Electronic apparatuses to which the light emitting device of theinvention can be applied include a television apparatus (a televisionset or a television receiver), a camera such as a digital camera and adigital video camera, a mobile phone unit (a mobile phone), a portableinformation terminal such as a PDA, a portable game machine, a monitor,a computer, a sound reproducing device such as a car audio system, animage reproducing device equipped with a recording medium such as a homegame machine, and the like. Specific examples thereof are described withreference to FIGS. 8A to 8F.

FIG. 8A illustrates a portable information terminal applying the lightemitting device of the invention, which includes a main body 9201, adisplay portion 9202, and the like. According to the invention, accurategray scale display can be performed.

FIG. 8B illustrates a digital video camera applying the light emittingdevice of the invention, which includes display portions 9701 and 9702,and the like. According to the invention, accurate gray scale displaycan be performed.

FIG. 8C illustrates a portable terminal applying the light emittingdevice of the invention, which includes a main body 9101, a displayportion 9102, and the like. According to the invention, accurate grayscale display can be performed.

FIG. 8D illustrates a portable television apparatus applying the lightemitting device of the invention, which includes a main body 9301, adisplay portion 9302, and the like. According to the invention, accurategray scale display can be performed.

FIG. 8E illustrates a portable computer applying the light emittingdevice of the invention, which includes a main body 9401, a displayportion 9402, and the like. According to the invention, accurate grayscale display can be performed.

FIG. 8F illustrates a television apparatus applying the light emittingdevice of the invention, which includes a main body 9501, a displayportion 9502, and the like. According to the invention, accurate grayscale display can be performed.

As set forth above, the light emitting device of the invention can beapplied to various electronic apparatus.

EMBODIMENT

In this embodiment, a current of a cathode (a cathode current) of alight emitting element was measured where a voltage of an anode (ananode voltage) thereof was set at 8 V and the number of rows forfull-white light emission with 1 bit was changed. Then, periods forinputting a signal into pixels of 320 rows (writing operation periods)were set at 1 μs, 500 ns, and 250 ns respectively, and the cathodecurrent was compared in the respective periods between the case where anerasing signal was inputted only once (shown by a dotted line) and thecase where the erasing signal was inputted two times (shown by a fullline, and the second input started with a delay of 20 rows).

In FIG. 10, the x-axis indicates the number of rows for light emission(320 rows) while the y-axis indicates the cathode current. It is idealthat the number of rows for light emission and the cathode current beproportionate to each other. However, in the case where an erasingsignal is inputted only once, as the writing operation period isshorter, the cathode current at the small number of rows for lightemission becomes larger as shown by the dotted lines; it means thatlow-gray scale display is not performed accurately. On the other hand,in the case where the erasing signal is inputted two times, therelationship between the number of rows for light emission and thecathode current is nearly an ideal proportional relationship as shown bythe full lines.

As set forth above, in the case where the input of an erasing signal issmall, namely in the case where the period for inputting an erasingsignal is short, the capacitance holding a gate-source voltage of thedriving transistor varies due to the capacitor (C_(EL)); and its effectis increased in particular when the writing operation period is short,and in addition, when the number of rows for light emission is small aswell. Accordingly, as the writing operation period is shorter, a drivingmethod for inputting an erasing signal two or more is more suitable.

This application is based on Japanese Patent Application serial no.2004-278492 filed in Japan Patent Office on 24th Sep. 2004, and theentire contents of which are hereby incorporated by reference.

1. A driving method of a light emitting device comprising a step ofproviding n subframe periods (n is a positive integer) in a frameperiod, wherein each of the subframe periods comprises a writingoperation period; and wherein at least one of the subframe periods has aplurality of periods for inputting an erasing signal.
 2. A drivingmethod of a light emitting device comprising a step of providing nsubframe periods (n is a positive integer) in a frame period, whereineach of the subframe periods comprises a writing operation period; andwherein at least one of the subframe period has a plurality of writingoperation periods.
 3. A driving method of a light emitting devicecomprising a step of inputting a video signal and a erasing signal toperform a digital gray scale display, wherein a period for inputting theerasing signal is longer than a period for inputting the video signal.4. A driving method of a light emitting device comprising a step ofinputting a video signal and a erasing signal to perform a digital grayscale display, the light emitting device comprising: a switchingtransistor, wherein a source electrode or a drain electrode thereof isconnected to a signal line and a gate electrode thereof is connected toa scan line; a driving transistor, wherein a gate electrode thereof isconnected to the switching transistor; and a light emitting elementconnected to a source electrode or a drain electrode of the drivingtransistor, wherein a period for inputting the erasing signal is longerthan a period for inputting the video signal.
 5. A driving method of alight emitting device comprising a step of inputting the video signaland a erasing signal to perform a digital gray scale display, the lightemitting device comprising: a switching transistor, wherein a sourceelectrode or a drain electrode thereof is connected to a signal line anda gate electrode thereof is connected to a scan line; a drivingtransistor, wherein a gate electrode thereof is connected to theswitching transistor; a erasing transistor for discharging a chargecorresponding to a gate-source voltage of the driving transistor; and alight emitting element connected to a source electrode or a drainelectrode of the driving transistor, wherein a period for inputting theerasing signal is longer than a period for inputting the video signal.6. A driving method of a light emitting device comprising a step ofinputting the video signal and a erasing signal to perform a digitalgray scale display, the light emitting device comprising: a switchingtransistor, wherein a source electrode or a drain electrode thereof isconnected to a signal line and a gate electrode thereof is connected toa scan line; a driving transistor, wherein a gate electrode thereof isconnected to the switching transistor; a erasing transistor fordischarging a charge corresponding to a gate-source voltage of thedriving transistor; a transistor connected to the driving transistor inseries and having an fixed gate potential; and a light emitting elementconnected to a source electrode or a drain electrode of the drivingtransistor, wherein a period for inputting the erasing signal is longerthan a period for inputting the video signal.
 7. A driving method of alight emitting device comprising a step of inputting a gray scale signalto perform an analog gray scale display, wherein the gray scale signalis inputted plural times.
 8. A driving method of a light emitting devicecomprising a step of inputting a gray scale signal to perform an analoggray scale display, wherein a period for inputting the gray scale signalin the case of low-gray scale display is longer than that in case ofhigh-gray scale display.